Beta Aquarii (β Aqr, β Aquarii) is a double star in the constellationAquarius. It has the traditional name Sadalsuud, from an Arabic expression سعد السعود sa‘d al-su‘ūd, the "luck of lucks". Other spellings that are sometimes encountered are Sad es Saud, Sadalsund, and Saad el Sund.

In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Nir Saad al Saaoud, which was translated into Latin as Lucida Fortunæ Fortunarum (rather identic with R.H. Allen), meaning the brightest of luck of lucks.[11]

X-ray emissions from the corona of this star have been detected using the Chandra X-ray Observatory; among the first such detections of X-rays for a G-type supergiant. A secondary X-ray source discovered near Beta Aquarii probably has an extragalactic origin.[9] This star belongs to a group of three intermediate mass stars with a space velocity that is carrying them perpendicular to the plane of the galaxy. The other members of this grouping are Alpha Aquarii and Eta Pegasi.[9]

Sadalsuud, when translated, means the "Luckiest of the Lucky" or the "luck of lucks". In the context of older worldviews (i.e.Egyptian, Persian and Islamic mythology), Sadalsuud relates to the rising of the Sun when winter has passed (March) and the season of gentle, continuous rain has begun.[14] Hence the myth of "luck" or "good fortune" was seen as closely aligned with the essence of spring itself, the burgeoning of new life, and by extension agriculture, which in all societies is the very foundation of prosperity or "good fortune". This mythological view of "the luck of the lucks" also belongs to the 22d Manzil(Arabic Lunar Mansion), which included the two stars Bunda (ξ Aqr) and c Cap.[14][15]

One of the many illustrations[16] from Gerardus Mercator - in this case of Aquarius, the water-bearer, overlooking Capricorn (the mountain-goat) and pouring water into the mouth of the southern fish (Pisces). Sadalsuud is the star in the left shoulder, just above Deneb Algedi (δ Cap)

β and ξ Aquarii also constitute the Persian lunar mansion Bunda and the similar Coptic mansion Upuineuti, the meaning of which is "the Foundation".

In Chinese mythology, β Aqr alone marks the sieu (Chinese Lunar Mansion) Heu, Hiu, or Hü, "the Void", anciently Ko, the central one of the seven sieu which, taken together, were known as Heung Wu, the Black Warrior, in the northern quarter of the sky.[14] As such, Sadalsuud is an expression of the feminine archetype, the Yin or "Void" (Cosmic Mother), from which, many cultures have believed, creation itself (birth) emanates.

In Chinese, 虚宿 (Xū Sù), meaning Emptiness (asterism), refers to an asterism consisting of β Aquarii and α Equulei.[17] Consequently, β Aquarii itself is known as 虛宿一 (Xū Sù yī, English: the First Star of Emptiness.)[18]

Sadalsuud is found in Hindu texts as Kalpeny and in the context of the ancient Indian system of astronomy, Jyotisha Veda, is located in the 23rd NakshatraShravishthā, a lunar mansion which is ruled by Eight vasus - the "deities of earthly abundance" . On the Euphrates, Sadalsuud was known as Kakkab Nammax, the Star of Mighty Destiny; that may have given origin to the title of the manzil, as well as to the astrologers' name for it — Fortuna Fortunarum.[14]

"Sadalsuud" appears to be a single star to the naked eye, but when viewed with a telescope is seen to have two faint optical companions. The first has an apparent magnitude of 11.0. In 1947, the position angle was observed at 321 degrees with a separation from the Beta Aquarii of 35.4 arcseconds.[19][20] The second star has a magnitude of 11.6. Its position angle is 186 degrees with a separation from Beta Aquarii of 57.2 arcseconds.[19][20] As of 2008, there is no definitive evidence that the three stars form a ternary star system.[21]

^β Aqr as Nir Saad al Saaoud or Lucida Fortunæ Fortunarum (the brightest of luck of lucks) and ξ Aqr as Thanih Saad al Saaoud or Secunda Fortunæ Fortunarum (the second of luck of lucks). c Cap should be Thalath Fortunæ Fortunarum or Tertia Fortunæ Fortunarum (the third of luck of lucks) consistently, but Al Achsasi Al Mouakket was not designated the title for this star with uncleared consideration. Possibly according to the differences of opinion with R.H.Allen